1. Field of the Invention
The present invention is related to satellite-based ground-positioning systems and, more particularly, to a method for reducing frequency-related biases in a such a system.
2. Background Information
The Global Navigation Satellite System (GLONASS) is designed to utilize up to twenty four active satellites in earth orbit. Each Global Navigation System (GNS) satellite transmits signals centered on two discrete L-band carrier frequencies, the frequency bands denoted by L1 and L2. The L1 band, for example, includes twenty four transmitting frequencies separated by 0.5625 MHz. A positional system using GLONASS signals employs frequency-division multiplexing to differentiate between the signals of the various satellites. As these signals pass through the RF section of a GLONASS receiver, frequency-related biases are produced, both in the pseudorange and in the carrier phase observations. These biases can arise from, for example, antennas, cables, filters, and various other components in the front end of the receiver.
One method of reducing the effects of the biases is to directly calibrate the receiver RF section. Unfortunately, this method is very labor intensive and requires specialized equipment. A more preferable method is to calibrate the effects of the inter-frequency biases on the measurements and correct the pseudorange or carrier phase measurements accordingly. For example, the calibration procedure may call for calibrating receivers in pairs, or calibrating a single receiver using a multi-channel GLONASS simulator as a reference. However, such calibration procedures do not readily lend themselves to operation in the field. Further, such calibration methods suffer from the shortcoming that the accuracy of the derived frequency biases are based on the functional attributes of the reference receiver or the simulator. Moreover, the multi-channel GLONASS simulator is an expensive device, and may have uncorrectable biases of its own.
While the relevant art describes methods for calibrating global positioning devices, there remains a need for a calibration method that offer advantages and capabilities not presently found, and it is a primary object of this invention to provide such a method.
It is also an object of the present invention to provide a method of calibration which does not require use of supplemental, specialized equipment.
It is another object of the invention to provide such a method which can be performed in the field.
Other objects of the invention will be obvious, in part, and, in part, will become apparent when reading the detailed description to follow.
The invention results from the realization that a significant source of the inter-frequency pseudorange bias in a GPS/GLONASS receiver results from signal group delay in the IF filter. The disclosed calibration procedure includes the steps of i) removing the dynamics of the GPS and the GLONASS satellites by using the ephemeris data for the satellites; ii) estimating the change in receiver clock offset by means of GPS satellite measurements, and iii) acquiring a set of two or more measurements for each satellite frequency of interest to estimate the corresponding inter-frequency bias. The disclosed method utilizes a receiver microprocessor to obtain pseudorange measurements while the GLONASS local oscillator in the receiver is placed at different frequencies. The disclosed method thus allows individual calibration of receivers and advance calibration of frequencies, where the calibration is valid for single point positioning as well as for differential positioning.